The invention relates to process control and, more particularly, to systems for analyzing processes to determine characteristics such as the non-linear behavior of process components.
Process control refers to a methodology for controlling the operational parameters of a process by monitoring one or more of its characteristics over time. It is used to ensure that the quality and efficiency of a process do not vary substantially during a single run or over the course of several runs. While process control is typically employed in the manufacturing sector, it also has application in service industries.
A process control unit, or "controller," typically operates by comparing values of a process characteristic--referred to as the controlled variable--with a target value to determine whether the process is operating within acceptable bounds. For example, in a process in which fluid flows at a constant rate from a tank that is continuously filled to constant volume, a controller monitors the liquid level and, if necessary to prevent the tank from running dry or overflowing, adjusts an inlet valve to increase or restrict inflow to the tank.
Another example is a process for maintaining a constant temperature in a room or building. In such a process the controller adjusts a thermostat to compensate for changes in heat flow caused by factors such as a rise or drop in the ambient temperature outside the building, or a sudden increase or decrease of the number of people in the building.
Process control systems are generally composed of transducers, controllers, and controllable components (e.g., valves, thermostats and the like). These elements are typically arranged in feedback loops that are well known in the art.
To simplify design problems, process control system designers generally assume that controllable components are linear. However, many real-world components actually behave according to non-linear functions. Further, even linear components can sometimes behave according to non-linear functions when they are installed in process control systems and are subject to well known phenomena such as "variable pressure drop." Ignoring these inherent non-linearities results in undesired error in the process control.
Once a controllable component is installed in a process control system, it is desirable to characterize the non-linearity of the component, at least in a neighborhood about some nominal operating point. As I described in F. G. SHINSKEY, PROCESS CONTROL SYSTEMS (3rd ed. 1988) ch. 2, it is a good approximation to characterize the non-linear behavior of a controllable component as belonging to one of a family of hyperbolic curves having the equation ##EQU1##
where f represents the component's output (expressed in percent), x represents the component's input (expressed in percent), and z is a parameter that characterizes the particular component's non-linearity.
This family of curves can be thought of as a universal characterizer since they provide a generic shape indicative of the typical behavior of non-linear controllable components. However, in the current state of the art there is no simple method for determining a value for the parameter, z, that will accurately characterize a particular controllable component.
Once a controllable component's non-linearity has been characterized, it is desirable to compensate for that non-linearity. Further, since controllers are generally designed to control linear components, it is desirable to compensate for the non-linearity so that standard linear control can be applied to the non-linear controllable component.
While many useful components exhibit compensatable non-linear behavior, others are simply defective. Some defective controllable components exhibit a form of behavior known as "dead-band" which is well known in the art. Rather than attempting to characterize and compensate for their defective behavior, it is preferable to identify such components and replace them.
In view of the foregoing, an object of the invention is to provide improved methods and apparatus for characterizing and compensating for non-linear components.
More particularly, an object of the invention is to provide a such methods and apparatus for compensating for (i.e., linearizing) non-linear components in a process control system.
A further object of the invention is to provide such methods and apparatus for determining a value of the parameter, z, that will accurately characterize the non-linearity of a controllable component.
Yet another object of the invention is to provide a method and apparatus for identifying controllable components of a process control system that exhibit deadband.